I want to build a dehumidification cabinet for my guitars and need advice on what parts to get.

Known parts list (have or will get):

Arduino Mega

Temperature sensor

Humidity sensor

Peltier cooler

Heatsink

Ventilation fan (for inside cabinet)

LCD display

Buttons to set desired humidity

Water container

Wooden cabinet

Need:

A suitable power supply along with whatever components are needed to get the right voltage and current to peltier cooler and case fan. The peltier cooler I'm looking at calls for a max voltage input of 29.8 VDC and a max current of 6A (4.2A recommended). The fan is a standard 12 VDC PC case fan.

A way to control the peltier cooler's temperature so condensation occurs without frosting over.

A component to turn the peltier cooler on and off so my sketch can maintain humidity at desired level.

There are two different and conflicting issues here. The first is that a traditional PWM scheme is a bad idea because it will be inefficient. As you increase the voltage, or current through a Peltier the cooling output will increase reach a peak and then decrease due to the additional power dissipated in the Peltier. In a PWM controll scheme you are emulating a lower supply voltage / current by pulsing a higher one on and off. But in this case the higher your input voltage is the less efficient the device will be, so you are much better off having a lower steady input voltage than a higher pulsed input voltage from an efficiency standpoint. That said, if your Peltier is sufficiently oversized you might be able to just write off the efficiency loss.

The other issue with PWM is thermal cycling. Anything heated and cooled enough times will be mechanically harmed by the effects of differential expansion and contraction of materials. What you want to avoid is a rapid thermal cycling of the Peltier device. If you PWM quickly (faster than 10 Hz) no appreciable temperature change will occur in the device during the cycle and you should be fine.

If you can capacitivly filter your PWM signal so that it is essentially a DC signal you will solve both of these issues at one time, so that is the best solution, but a quick PWM with a maximum voltage that is still in the good efficiency range would probably work.

I think it means that you need to have a power supply capable of supplying the maximum amperage the device needs, without stressing the power supply. A 6A power supply supplying 6A all the time will get hot, and not last as long as you might wish. A 6A power supply supplying 4.2A all the time will be a lot cooler, and last a lot longer.

Zorb-its are good for maintaining average humidity levels, but not so much for use as dessicants. Average humidity is fairly high here in Puerto Rico, meaning Zorb-its would only serve to maintain our rather high average humidity.

There are two different and conflicting issues here. The first is that a traditional PWM scheme is a bad idea because it will be inefficient. As you increase the voltage, or current through a Peltier the cooling output will increase reach a peak and then decrease due to the additional power dissipated in the Peltier. In a PWM controll scheme you are emulating a lower supply voltage / current by pulsing a higher one on and off. But in this case the higher your input voltage is the less efficient the device will be, so you are much better off having a lower steady input voltage than a higher pulsed input voltage from an efficiency standpoint. That said, if your Peltier is sufficiently oversized you might be able to just write off the efficiency loss.

The other issue with PWM is thermal cycling. Anything heated and cooled enough times will be mechanically harmed by the effects of differential expansion and contraction of materials. What you want to avoid is a rapid thermal cycling of the Peltier device. If you PWM quickly (faster than 10 Hz) no appreciable temperature change will occur in the device during the cycle and you should be fine.

If you can capacitivly filter your PWM signal so that it is essentially a DC signal you will solve both of these issues at one time, so that is the best solution, but a quick PWM with a maximum voltage that is still in the good efficiency range would probably work.

This is very good info. A PWM signal filtered by a capacitor looks like the best approach for temperature control, but what about humidity control on top of it? It seems to me that switching the PWM signal on and off according to measured humidity levels would lead to the same expansion and contraction issues I am trying to avoid. What, then, would be the best way to accomplish the same effect?

So what you want to do is set the cold side of the peltier to the dewpoint so that you can collect the condensation and remove it from the cabinet.

You're going to attach some sort of aluminum plate/heat sink to each side of the peltier. The hot side will need a fan to turn on whenever the peltier is active. On the cold side you will also need a temperature probe of some type (e.g. LM35 or DS18B20) to measure the temperature of that side. Use the PID library to control the power on/off to the peltier (via relay or mosfet, preferably mosfet) to keep the dewpoint temperature of the cold side stable.

There's no PWM needed here, or putting it another way there's no reason why you'd need to switch the peltier on/off more than a couple times a second to keep a reasonably stable temperature.

There's no PWM needed here, or putting it another way there's no reason why you'd need to switch the peltier on/off more than a couple times a second to keep a reasonably stable temperature.

What about thermal stresses, as noted by jroorda? His reply is consistent with this FAQ, which states:

Quote

"Significant precautions must be employed with PWM, however. First of all, the PWM should be at a high enough frequency to minimize thermal stresses to the TE devices. While we like to keep the frequency in the low killihertz (Hz) range, in many applications these days we must compromise at around 120 Hz for the sake of electromagnetic compatibility. Another important issue is the potential for generating electro-magnetic interference (EMI) in the wiring to the TE device. If you are using PWM, you may need to shield your power wiring or keep it away from any sensitive electrical signals."

That FAQ and all of their technical specifications provide pretty much no detail as to thermal stress considerations, and if it were a big issue to be concerned about I would expect that they would list some specification as to the number of switching cycles it can handle. It also contradicts itself with its discussion of using mechanical relays by suggesting that the relay fails before the peltier junction does? After digging around with Google I'm not denying that PWM would be preferable, but I still can't find any decent numbers as to how much of a concern this is.

Stick with the K.I.S.S. principle. A peltier junction is ~$10, and if it does wind up failing too quickly then you can revisit how you're driving it. Drive it at a low voltage and tweak the voltage a bit until you get a reasonably low switching rate. You don't really need that much cooling; At 70F (room temperature) the dewpoint is only 50F for 50% humidity and that should be a much much lower power requirement than these devices are capable of.